Three suspects who have admitted to breaking into the Y-12 National Security Complex appeared in court on Tuesday.
Michael Walli, Megan Rice, and Greg Boertje-Obed are charged with sabotage and trespassing after cutting through security fences and vandalizing property in July 2012.
In court, attorneys argued over whether a witness for the defense should be allowed to testify.
The defense wants to use testimony from former US Attorney General Ramsey Clark to prove the suspects' actions were justified.
During questioning, Clark said it is "reasonable" to say that the suspects had no other legal alternative than their actions. He also indicated he believe the suspects' actions didn't harm national security.
During cross-examination, Clark said he believes Y-12's operations are unlawful.
A federal judge will issue a decision on whether Clark may be admitted as a witness within a week.
Jury selection for the trial is set to begin on May 6.